1. Field of the Invention
The present invention generally relates to a sewing machine or system and particularly to such a sewing system which has the function to detect an abnormality of a sewing device thereof.
2. Related Art Statement
There is known a multiple-head embroidery sewing system having a plurality of sewing heads. The sewing system includes a control device which operates for reciprocating a needle bar or sewing needle provided in each sewing head while moving an embroidery frame supporting a work sheet in a plane defined by an X and a Y axis according to embroidery data indicative of stitch positions in the X-Y plane where the sewing needle penetrates the work sheet to form a series of stitches. Thus, the sewing system can concurrently produce a plurality of identical embroideries on the work sheet.
Each of Japanese Unexamined Patent Applications published under No. 62(1987)-57588, No. 62(1987)-57585 and No. 2(1990)-91264 discloses an example of the multiple-head embroidery sewing system. Each prior sewing machine has the function to calculate, before starting a sewing operation, a sewing time duration necessary to finish the sewing operation of forming a series of stitches as an embroidery. Based on the thus calculated or estimated sewing time duration, an operator can make his or her production plan. The control device of each sewing system automatically (a) determines an actual sewing speed (e.g., number of rotations per unit time of a main-shaft motor) at which to actually form each one of the series of stitches, based on (i) a length of each one stitch which is determined by utilizing embroidery data and (ii) a maximum sewing speed (i.e., upper limit of the rotation number of the main-shaft motor) which is set in advance by the operator, (b) calculates, based on the thus determined actual sewing speed, a time period necessary to form each one stitch, and (c) sums the time periods for all the stitches so as to determine the sewing time duration as a whole necessary to form the series of stitches as the embroidery.
In the above-indicated sewing system, however, a sewing thread supplied to the needle bar or sewing needle often breaks during the sewing operation. The frequency of occurrence of thread breakage varies depending upon actual sewing speeds, lengths of stitches, or thicknesses (i.e., thickness values) of sewing threads. However, thread breakage itself will occur in any sewing machine. Additionally, in the event that a drive mechanism of the sewing machine suffers from maladjustment or a failure, the frequency of thread breakage increases.
In general, an embroidery sewing system has a thread-breakage sensor which detects a breakage of a sewing thread supplied to a sewing needle. When the thread-breakage sensor detects a thread breakage, the sewing system informs an operator of the occurrence of thread breakage, by causing a buzzer to sound an alarm. At the same time, the sewing system stops the sewing operation. After the operator attends to the sewing system for passing the broken thread through the eye of the needle and operates a `restart` switch thereof, the sewing system resumes the normal sewing operation. The thread breakages result in deteriorating the quality of the embroidery produced, increasing the sewing time duration due to the lost time necessary to restore the sewing system from the thread breakages, and lowering the production efficiency of the sewing system.
However, in the above-indicated conventional manner of estimating the sewing time duration, the lost time necessary to restore the sewing system from one or more thread breakages is not taken into account. Therefore, if one or more thread breakages occur during a sewing operation to form a series of stitches, an actual sewing time duration necessary to form the series of stitches increases. Thus, in the conventional sewing-time-duration estimation method, the possibility of occurrence of thread breakage, or lost time necessary to restore the sewing system, is not taken into account. Thus, the sewing time duration estimated by the conventional method is not satisfactorily reliable.
In addition, the conventional sewing system only informs the operator of the occurrence of each thread breakage. Even if the cause of thread breakage is not essentially inevitable, for example, due to maladjustment of the drive mechanism of the sewing system or excessively high sewing speed which causes can easily be removed, the operator cannot understand that the sewing system is in such a situation. Consequently, thread breakages will occur again and again due to the same cause, thereby lowering the production efficiency of the sewing system.
Meanwhile, in the above-indicated conventional multiple-head embroidery sewing system, the operator may set, before starting a sewing operation, a desired tension of a sewing thread supplied to a needle bar or sewing needle in each sewing head, by turning a knob of a thread-tension adjusting device.
There are known other causes of the occurrence of thread breakage than the above-indicated maladjustment of the drive mechanism and excessively high sewing speed; such as the use of a sewing thread which easily breaks, maladjustment of mechanical parts of the sewing system, and excessively high thread tension. Two or more of those causes may combine to produce a thread breakage. In addition, the sewing thread may break due to a mere accidental cause, such as including a weak or defective portion of a low mechanical strength.
In the embroidery sewing system which forms a series of stitches according to embroidery data indicative of stitch positions where the sewing needle penetrates the work sheet, the manner of arrangement of the embroidery data may cause the embroidery frame to make an abrupt, complex movement at a certain timing or position, thereby excessively largely twisting the sewing thread. The sewing thread easily breaks at such timings or positions in the sequence of formation of the series of stitches. This is an intrinsic cause of the occurrence of thread breakage, and therefore the thread breakage will occur again and again at the same position in the sequence of formation of stitches.